


The Theory of Falling in Love

by kawaiisumi



Series: Matsuoka Gou Week 2018 [4]
Category: Free!
Genre: Birthday, F/M, Party, Surprise Party
Language: English
Status: Completed
Published: 2018-12-22
Updated: 2018-12-22
Packaged: 2019-09-24 16:20:45
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,603
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17103953
Author URL: https://archiveofourown.org/users/kawaiisumi/pseuds/kawaiisumi
Summary: All Rei agreed to was helping Rin plan a surprise birthday party for his younger sister, Kou. What he hadn't accounted for was his feelings for her beginning to grow and come to fruition until it was just impossible to ignore any longer. After all, the idea of love plausibly made no sense.Aka, Rei tries (and fails) to ignore his own irrational feelings for the one girl he cares about the most.





	The Theory of Falling in Love

**Author's Note:**

> I've been slacking on posting lately!! I'm so sorry! Here's my next installment for Matsuoka Gou week, combining the prompts for day 4 and 5, "Birthday" with "Ships"! I hope you enjoy it! Hopefully I'll catch up with the other prompts soon!

**The Theory of Falling in Love**  
_Aka, Rei tries (and fails) to disprove his own irrational feelings for the one girl he cares about the most._

“Rei… Are you sure this is going to work?” Rin’s eyes were surprisingly cool considering how warm toned his red irises are. Kou meant a lot to him, regardless of how he might act. But then again, Rei knew Kou well too. And this plan was perfect.

“Of course!” Rei said, waving his hand nonchalantly at Rin. In front of him was a well-designed map, with the words “Operation: Birthday Party” written in bolded letters at the top. Among the haphazardly drawn photos are clear instructions, colour coded and underlined.

According to Rin, Kou had never had a surprise birthday party. She had wanted one since she was young, but her mother had never had the time to plan one for her. Rin had also tried on many occasions, but his efforts seemed to fall flat every time. Kou either figured it out, or the plans didn't materialize right.

When Rin first approached Rei to ask him for help, Rei was surprised when it had nothing to do with swimming at all. It had to do with his other budding interest, who just happened to be Rin’s sister. Not that he could ever tell Rin that. He very much wanted to keep his head on his shoulders, lest Rin bite it off. “Rin-san, I’ve read _multiple_ articles online to the success of teenage birthday parties, _and_ I’ve provided you a list of activities to complete with Gou-san while we prepare Haruka-senpai’s house for the party.” Rei pushed his glasses up the bridge of his nose purposefully. “Nothing can go wrong!”

But of course, Rei always had to jinx himself.

“Trains from Toyooka have been cancelled because of an accident on the tracks,” Rin said to him over the phone, the following week. “I have to go pick her up if she has any chance of making it to Kou’s birthday party tonight.”

Rei, who was sitting in Haru’s house with the rest of the Iwatobi gang, pursed his lips. He tried to think of another solution. If Rin was away retrieving his and Kou’s mother, someone else would have to distract her for the day. Who would be a good choice? Haru was out, considering he’d agreed to do a majority of the cooking, Makoto was a terrible liar, and Nagisa… Rei shivered. Knowing Nagisa, he and Kou would never make it back to the party on time. That left only one solution, he himself would have to keep Kou company all day. At least, until Rin and her mother had made it back to Iwatobi.

“Don’t worry Rin-san, I’ll take care of it,” Rei said firmly, “I can handle this!”

When he hung up the phone, Haru shrugged, going back into the kitchen to continue cooking. Makoto followed suit, leaving Nagisa and Rei sitting at the coffee table. Rei retrieved his copy of the list he had provided for Rin. The schedule consisted of many places Rin was supposed to take Kou to keep her distracted until that evening. At the time he had made it, it didn’t seem so bad, considering it was supposed to be Rin taking her. But now that it was him... The whole thing felt almost like the perfect plan to the perfect date, and despite all logic (because what other purpose did love serve besides procreation?) he couldn’t help but feel his heart skip at the prospect of a day alone with Kou.

Nagisa stared at him, a sly smile on his face. “Hm… So Rei-chan is going to spend the whole day with Gou-chan?” He asked, wiggling his eyebrows. 

Rei could feel his face flushing. “It’s not like that Nagisa-kun!” He said rather forcefully, rising from the table. He collected his things, putting them into his knapsack while Nagisa continued to giggle at him. “We must keep Gou-san out of the house in order for everything to go perfectly. I trust no one more than myself to fulfil that duty!”

Nagisa snorted, though allowed Rei the smidge of confidence he had left in himself. “Good luck Rei-chan! We’ll be waiting for you here to bring Gou-chan tonight.”

With Nagisa’s (weak) vote of confidence, Rei left Haru’s house, walking down the steps toward the train station. It wouldn’t be so hard. He would take Kou to the mall. They’d look around and window shop until they found the things they needed for the swim club at the athletic store. Rei got onto the train, reviewing the itinerary as he went. Next, they would visit the bakery to pick out a cake. Kou was supposed to be under the impression that they were picking a cake to share at home with her mother. Rei mused that the idea would seem much more plausible had it been Rin taking Kou, but he had no time to fix the inconsistency now. The train came to a halt at the station 2 stops over, and Rei hastily got off the train, speed walking now toward the Matsuoka house. 

When he knocked on the door, Kou greeted him. She was still getting dressed, wearing only a camisole and a pair of shorts. “Ah! Rei-kun!” She said in surprise, opening the door more for him to come inside. “What a surprise! I thought you were onii-chan!”

Rei flushed, willing himself to look away while she was dressed so… _scarcely_. “R-R-Rin-san said that he needed to pick up your mother from Toyooka, so he cannot do errands with you today.” The words sounded better in his head, but he hoped Kou got the message. He felt like his eyes were burning holes into the floor, as he stared literally anywhere but Kou.

Shameless as always, she didn’t seem to catch on, though agreed to go on errands with him instead of her brother. Kou excused herself to finish getting ready, leaving Rei to wait on the living room couch. Rei had never been to Kou’s house before. He wasn’t sure what he had been expecting, but the house was much more traditional than he expected. It was a lot like Haru’s house, with a few renovative adjustment. Nonetheless, it felt like a cozy family home. He wondered what Kou must have been like as a young girl, having grown up in this house. He wondered what she must have done for fun or occupy her time…

Rei shook his head, urging himself to think about the goal in mind. He had to keep Kou away from Haru’s house _and_ keep her unsuspecting for the next few hours. The nerves jittering deep in his chest were not helping. He couldn’t help but feel frustrated at how irrational his emotions were. It wasn’t as though he didn’t believe in love and affection per say. To him, it seemed like a rather integral part of human evolution. If men and women did not find attraction in each other, the likelihood of producing offspring to continue the human race plummeted.

Yet, he had no rationale to explain why his heart hammered when Kou came down the stairs in a pale yellow sundress. After all, it wasn’t as though he needed a child anytime soon. “Are we good to go Rei-kun?” Kou asked sweetly, putting on her sandals by the front door.

Rei stood, nearly hitting his shin against the low coffee table. “Um, yes!” He stuttered, joining her at the door.

Kou opened the door, allowing him to step out first. “Thank you for coming,” she said, locking the door behind them. “It’s weird that onii-chan sent you. It’s not like I haven’t gone and done errands by myself before.”

“Ah… Well that’s…” Rei tapered off awkwardly, unsure of what to tell her. It wasn’t often that he found himself unsure of what to say, especially when he had a plan to go off of, but something about Kou made his mind feel empty.

“Oh well,” Kou smiled, grabbing hold of his forearm to drag him along to the train station. “Errands aren’t going to finish themselves, what’s first on the list?”

“The sports store for some new pool equipment,” Rei said, fumbling to adjust his glasses. He snuck a peek at her small hand gripped tightly to his arm, finding it difficult to concentrate when he could feel her skin against his.

When they arrived at the train station, Kou let go of him, and Rei exhaled a breath he didn’t even know he had been holding in. The whole way to the store, Kou chatted aimlessly about one thing or another. Rei couldn’t really remember what she was saying, but he could remember the way her cheeks lifted high when she smiled. The corners of her eyes creased softly, melting into her skin when she laughed. It was a sight Rei couldn’t describe other than beautiful; in one of those ways that pertained to no theory or calculation, much like Haru’s swimming. If Rei found himself full of desire to swim just like Haru, he found himself desiring to be the one to put such a tender smile on Kou’s face.

“Rei-kun, this is our stop,” Kou said, interrupting his thoughts, as she rose to her feet. It was true. From the window, the large sports store attached to the mall was visible from the next street over.

For a Sunday, the streets were very crowded, two rather tight lines of foot traffic lining the sidewalks, one going each way. “There sure are a lot of people,” Kou mused. Whether she was talking to Rei or herself, he was unsure.

“I suppose so,” Rei said, “If one takes into account the calendar year for which holidays are closest to today perhaps one could understand why so many-” Kou interrupted by quickly grabbing onto his shirt, pulling the dark blue fabric back. Rei felt himself jump, stealing a glance at her by turning his head.

“Ah! Sorry!” Kou apologized, quickly letting go. “We almost got separated when someone bumped into me, so I grabbed your shirt impulsively.”

Rei tried to swallow but found his mouth far too dry to do so. “It’s okay,” he said lamely, “You should… You should stick close. We wouldn’t want to get separated.” Rei couldn’t bear to look her in the eye, keeping his face forward to prevent her from seeing the heat that burned his cheeks. But nonetheless, Kou reached out to hold his hand. He jerked, not quite expecting that to be her first choice of keeping them together. Yet, there was something comforting about her small hand intertwined with his, so soft and warm, with the desire to hold her close and keep her safe welling up deep in Rei’s chest. It only made sense, he thought to himself. The male species was designed to look out for females. It had something to do with Darwin’s theory of evolution and hormones and maybe some other stuff… But Rei found his head buzzing with too many conflicting emotions to fully process the thoughts going through mind.

The sports store was thankfully less crowded, and Kou let go of his hand to begin browsing around. Part of Rei was relieved he no longer had to think about the physical contact between the two of them, but the other part of him craved it deeply. It made sense, he told himself. Humans were predisposed to be social creatures. Human contact was one of the many ways that humans connected with others in order to network and forward their survival.

“Rei-kun, it looks like they have what we need over there,” Kou said, wandering away to grab what they needed from the shopping list. Their time in the store was short lived. Rei was surprised to find out that Kou was a rather forward shopper. She didn’t take time to dilly dally around. She simply got what she needed and moved on. Rei could appreciate that. He had never understood the pull of window shopping anyway.

Regardless, they finished their shopping at least an hour before the scheduled time Rei had written down on his list of things to do. “Next is buying a cake from the bakery,” Rei read, as Kou checked over all the receipts and compared them to the swim club’s budget.

“The bakery is near Haruka-senpai’s house,” she said, “maybe we should bring this stuff to Haruka-senpai’s place so we don’t have to carry it the whole way.”

The suggestion made sense. Logically, it provided the best means of resources, considering the two of them would be free to carry any other things they might acquire along the way. However, going to Haru’s house was completely out of the question. If memory served Rei correctly, Nagisa and Makoto would be in the backyard blowing up balloons. “I don’t think Haruka-senpai is home!” Rei said abruptly, causing Kou to flinch for a moment. “I mean… I think Haruka-senpai said he was going to… to… Toyooka with Rin to pick up your mother.”

That was the worst possible lie he could have told, but what was done was done. Kou’s eyebrows furrowed for a moment as she processed the information, though the confusion quickly melted away. “Onii-chan and Haruka-senpai always do such weird things together,” she said with a shrug. “Let’s go to the bakery first and then drop it all off at my house then.”

To Rei, the train ride to the bakery was awkward. Though perhaps he himself was making it awkward. After all, social psychologists did say that one perceived a situation involving other people based on the characteristics of the person themselves rather than the situation they were in. It was called the fundamental attribution error after all. Perhaps the situation wasn’t awkward because Kou thought he was awkward but he simply perceived it as her believing he was awkward… Despite how convoluted that all sounded.

When they arrived at the bakery, it was almost half past 2, meaning that Haru would have almost finished the food preparations and would begin helping decorate the interior of the house with banners and streamers. Rei checked his watch. Rin should have just arrived to pick his mother up in Toyooka. “Look Rei-kun, they just finished making fresh bread!” Kou said excitedly, watching as the baker slowly sliced into the loaf. The aromatic aroma of spices made Rei’s mouth water in anticipation, while the baker spread slices of chestnut filled bread in the display case. “I’ll take two slices of bread please!” Kou said, fishing out her wallet as the baker quickly buttered the two slices.

“Gou-san what are you doing?” Rei asked, trying to pull out his wallet to pay for it instead. Kou waved him off, sliding a few bills onto the counter.

“It’s on me Rei-kun,” she said sweetly, “Think of it as a thank you for accompanying me today!”

He couldn’t help but watch her, humming as she browsed the bakery, trying to decide between a chocolate cake and a strawberry cake. Rei could do nothing but quietly eat his slice of bread and trail behind her. The bread was warm and nutty, fresh enough that the cream butter just soaked into it. “Do you think I should get the chocolate cake?” Kou asked. “Onii-chan doesn’t like chocolate, but I do, and it’s for my birthday after all. What do you think Rei-kun?”

Rei crammed the rest of the bread in his mouth, almost choking. “Ah… It’s up to you Gou-san. As you said, it’s your birthday. Perhaps if you’re worried about what your brother will think you could buy a vanilla cake? Studies show that those who do not like sweets much prefer the demure taste of vanilla to-”

Kou interrupted him by laughing, her nose wrinkling cutely as she did. Rei felt his face warm. “You’re so funny Rei-kun,” she said, ultimately pointing to the chocolate cake and letting the baker package it into a pink cake box. “I’m sure onii-chan will be okay with a chocolate cake. I’ll just make coffee too, so he has something more bitter to wash it down with.”

Rei nodded sheepishly, staring at his feet. Once Kou had paid, he helped her carry all of their purchases back to the train station. The sun had begun to set, and Rei wondered what the plan would be now. Despite all the careful and meticulous planning he had done to ensure the day went perfectly, he had forgotten one tiny detail.

He had no idea how he was going to get Kou to go to Haru’s house once they got back to her place.

There was silence on the walk back. Kou’s footsteps fell in line with his, the sun casting deep shadows of their silhouettes across the pavement. “Neh, Rei-kun… Would you like to come in for some tea?” Kou asked, once they had arrived at her front door.

Rei would never understand what possessed her to ask, nor would he know what encouraged him to agree, but he found himself again sitting in her living room as she moved around the kitchen. Kou returned with a tray, balancing the tea pot and two mugs. “I’ve only got black tea, I hope that’s okay,” she said, taking a seat beside him on the couch and pouring two cups. Rei nodded, accepting the mug from her and taking a tentative sip.

“Um… Gou-san,” Rei said, awkwardly turning his tea cup in his hand. “I should… We should get going…”

Kou raised an eyebrow, her lips turning up into a smile. “Leaving so soon? What, are you scared of me?”

Despite all logic, Rei found himself blushing embarrassedly. “It’s not that! It’s just getting late and all so perhaps I should take my leave now… But maybe I should wait for Rin-san to come back so you’re not here by yourself…”

Kou laughed softly, leaning against him. “That’s very thoughtful of you.” There was a moment of stillness, so quiet Rei was afraid Kou could hear his heart thundering in his chest.

“Rei-kun, can I ask you a question?”

Rei took a gulp of his tea, polishing off the drink. He held the empty cup so tight in his hand, the pads of his fingers were turning white. “What is it Gou-san?”

“Why did you go out with me today?”

The question was sincere. He could hear that much in her voice. Though, he wondered why that might matter at all. He took a deep breath turning his head to get a good look at her. Her face was flushed, just like his. In fact, Rei mused he’d spent a majority of the day caught in his head, he hadn’t quite had the time to contemplate her own expressions. “I… Well…” Rei felt as though all the words in the world had gotten jammed in his throat. “I simply… Chose to accompany you in lieu of your brother,” he said awkwardly, unable to be honest with himself or her. 

The fact of the matter was, no matter how much he tried to deny it, he couldn’t escape the obvious truth right in front of him. He loved Kou. The longer he spent with her, with every day that passed by, despite all logic and theory, he loved her. He was unsure what to do with this revelation, sitting there alone with her, but the look in her eye gave him some sort of clue. “Are you sure it was just because of that?” Kou asked quietly, her head moving closer to his, her body leaning closer into his until her chest was flush against him.

Rei shook his head wordlessly. He set down the cup on the coffee table, taking a deep breath. “I… Logically, I can only explain my behaviour with the notion that I care about you. Quite deeply.” Rei cursed inwardly at himself for being so unbearably awkward, but Kou didn’t seem to mind. Her lips turned up in a smile and she leaned forward to brush a soft kiss against his cheek.

“I care about you too Rei-kun,” she said. It was a simple response, but they both understood the weight of the words exchanged. They sat there for a while saying nothing to each other. Rei’s hand gently brushed against the soft skin of her wrist, causing her to look up at him expectantly. “Gou-san, would you be so kind as to indulge me this one time?” Rei asked quietly, his other hand gently caressing the side of her face.

She nodded, allowing him to tilt her chin upward so his lips could meet hers at the most logical angle. Rei’s first thought was that her lips were soft. They were reminiscent of strawberry lip gloss and sugar. The idea of exchanging saliva with someone else, with all the possibilities of swapping germs, had previously detested Rei. But sharing a kiss like this with Kou melted all those fears away.

They separated only when Rei’s phone rang, with caused Rei to jump, scrambling to retrieve it from his backpack.

“Rei, where the _hell_ are you?” Rin’s voice poured through the phone irritably. Rei looked onward at the clock on the mantle, realizing he was supposed to have brought Kou over to Haru’s house over an hour ago.

“Erm, we’re at your house,” Rei said. He glanced at Kou, who was seemingly begin to put the metaphorical pieces into place in her head. It wouldn’t be long until the realization completely dawned on her.

Well, there was always next year.

**Author's Note:**

> I hope you enjoyed this fic! Personally, I found it a lot of fun to write. I think the more I practise, the more I begin to understand how to write Rei's character, which while is one of my favourites, is one I find particularly difficult to write. Let me know what you thought of the whole fic!


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